For an increasing number of applications of LCDsxe2x80x94for example for use in automobiles, in which a temperature range of from xe2x88x9240xc2x0 C. to 100xc2x0 C. can readily occur, but also for portable equipment, such as mobile telephones and notebook PCsxe2x80x94there is a need for liquid-crystal mixtures which on the one hand have a very broad working-temperature range and on the other hand have the lowest possible threshold voltage.
There is therefore a continuing demand for novel, suitable liquid-crystal mixtures and mixture components. As described in Ichinose et al. (IDW 00, Abstr. LCT4-3) or in DE-A-10 050 071, materials are being sought in which high optical anisotropy (xcex94n) and low rotational viscosity coexistxe2x80x94with other parameters, such as, for example, high absolute values of the dielectric anisotropy (xcex94xcex5), likewise being required in addition to further applicationally relevant parameters.
The object of the present invention is therefore to provide novel components for use in nematic or cholesteric or chiral-smectic liquid-crystal mixtures which have high absolute values of the dielectric anisotropy combined with a favorable ratio of viscosity and clearing point. In addition, the compounds should to a large extent be light- and UV-stable and thermally stable. Furthermore, they should be suitable for achieving a high xe2x80x9cvoltage holding ratio (VHR)xe2x80x9d. They should furthermore be readily accessible synthetically and therefore potentially inexpensive.
The anthracene derivatives having 2-(4-subst.)styryl substituents (Helv. Chim. Acta 52, 2521 (1969)) or 2-(4-subst.)benzylidenamino substituents (Bull. Chem. Soc. Jpn. 50, 1009 (1977)), the known derivatives of 1,2,3,4-tetrahydroanthracene (J. Org. Chem. 56, 989 (1991)), the known mono- and difluoroanthracenes {Isr. J. Chem. 11, 791 (1973); Synlett 1998, 301}, tetrafluorotetrahydroanthracenes (J. Chem. Soc. Perkin Trans. 1 1972, 2372), tetrafluoroanthracenes (J. Fluorine Chem. 29, 417, 1985), the 9,10-diphenylanthracenes (JP-A 61 281 193), substituted anthracene (EP 915 144) and the cholesterol esters of anthracene (WO 88/00938), do not suggest to the person skilled in the art that fluorine-substituted derivatives of these ring systems are suitable as a component of liquid-crystal mixtures.
The present invention relates to compounds of the formula (I) and to liquid-crystal mixtures comprising these compounds 
in which:
L1, L2, L3, L4 and L5 are identical or different and are H or F, with the provisos that
a) at least one of L1, L2, L3 L4 and L5 must be F
b) if L1 is F, L3, L4 and L5 must be H
c) if L4 or L5 is F, L1 and L2 must be H,
R1 is a straight-chain or branched alkyl radical having from 1 to 16 carbon atoms or a straight-chain or branched alkenyl radical having from 2 to 16 carbon atoms, in which
a) one or more non-adjacent and non-terminal CH2 groups may be replaced by xe2x80x94Oxe2x80x94, xe2x80x94C(xe2x95x90O)Oxe2x80x94, xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94, xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94Oxe2x80x94, xe2x80x94C(xe2x95x90O)xe2x80x94 or xe2x80x94Si(CH3)2xe2x80x94, and/or
b) one CH2 group may be replaced by xe2x80x94Cxe2x89xa1Cxe2x80x94, cyclopropane-1,2-diyl, cyclobutane-1,3-diyl, cyclohexan-1,4-diyl or phenylene-1,4-diyl, and/or
c) one or more H atoms may be replaced by F and/or Cl,
R2 is hydrogen, F, Cl, CN, xe2x80x94NCS, CF3, CHF2, CH2F, OCF3, OCHF2, OCH2F, OCH2CF3, OCHxe2x95x90CF2 or a straight-chain or branched alkyl radical having from 1 to 12 carbon atoms or a straight-chain or branched alkenyl radical having from 2 to 12 carbon atoms, in which
a) one or more non-adjacent and non-terminal CH2 groups may be replaced by xe2x80x94Oxe2x80x94, xe2x80x94C(xe2x95x90O)Oxe2x80x94, xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94, xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94Oxe2x80x94, xe2x80x94C(xe2x95x90O)xe2x80x94 or xe2x80x94Si(CH3)2xe2x80x94, and/or
b) one CH2 group may be replaced by xe2x80x94Cxe2x89xa1Cxe2x80x94, cyclopropane-1,2-diyl or cyclobutane-1,3-diyl, and/or
c) one or more H atoms may be replaced by F and/or Cl and/or the -M2-A2-R3 group, in which
M2 is xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94CH2xe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94CH2xe2x80x94, xe2x80x94CF2xe2x80x94Oxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94CFxe2x95x90CFxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94CF2xe2x80x94CF2xe2x80x94, xe2x80x94(CH2)4xe2x80x94,
xe2x80x94OC(xe2x95x90O)CFxe2x95x90CFxe2x80x94 or a single bond,
A2 is 1,4-phenylene, in which one or two H atoms may be replaced by F, Cl, CN and/or OCF3 or up to three H atoms may be replaced by fluorine, 1,4-cyclohexylene, in which one or two H atoms may be replaced by CH3 and/or F, 1-cyclohexene-1,4-diyl, in which one H atom may be replaced by CH3 or F, or 1,3-dioxane-2,5-diyl,
R3 has the same possible meanings as R2, with the exception of -M2-A2-R3, 
Preference is given to the compounds of the formulae (Ia) to (It): 
in which
R4 is a straight-chain or branched alkyl radical having from 1 to 16 carbon atoms or a straight-chain or branched alkenyl radical having from 2 to 16 carbon atoms, in which one non-terminal CH2 group may be replaced by xe2x80x94Oxe2x80x94 and/or one CH2 group may be replaced by cyclohexane-1,4-diyl or phenylene-1,4-diyl,
R5 is hydrogen, a straight-chain or branched alkyl radical having from 1 to 12 carbon atoms or a straight-chain or branched alkenyl radical having from 2 to 12 carbon atoms, in which
a) one or more non-adjacent and non-terminal CH2 groups may be replaced by xe2x80x94Oxe2x80x94, xe2x80x94C(xe2x95x90O)Oxe2x80x94, xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94 or xe2x80x94Si(CH3)2xe2x80x94, and/or
b) one CH2 group may be replaced by xe2x80x94Cxe2x89xa1Cxe2x80x94, cyclopropane-1,2-diyl or cyclobutane-1,3-diyl, and/or
c) one or more H atoms may be replaced by F and/or Cl, or the -M2-A2-R3 group, in which
M2 is xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94CH2xe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94CH2xe2x80x94, xe2x80x94CF2xe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94CF2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94CF2xe2x80x94CF2xe2x80x94, xe2x80x94(CH2)4xe2x80x94, xe2x80x94OC(xe2x95x90O)CFxe2x95x90CFxe2x80x94, or a single bond,
A2 is 1,4-cyclohexylene, in which one or two H atoms may be replaced by CH3 and/or F, 1-cyclohexene-1,4-diyl, in which one H atom may be replaced by CH3 or F, or 1,3-dioxane-2,5-diyl,
R3 is hydrogen, a straight-chain or branched alkyl radical having from 1 to 12 carbon atoms or a straight-chain or branched alkenyl radical having from 2 to 12 carbon atoms, in which
a) one or more non-adjacent and non-terminal CH2 groups may be replaced by xe2x80x94Oxe2x80x94, xe2x80x94C(xe2x95x90O)Oxe2x80x94, xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94, or xe2x80x94Si(CH3)2xe2x80x94, and/or
b) one CH2 group may be replaced by xe2x80x94Cxe2x89xa1Cxe2x80x94, cyclopropane-1,2-diyl or cyclobutane-1,3-diyl, and/or
c) one or more H atoms may be replaced by F and/or Cl,
R6 is a straight-chain or branched alkyl radical having from 1 to 12 carbon atoms, in which
a) one or more non-adjacent and non-terminal CH2 groups may be replaced by xe2x80x94Oxe2x80x94, xe2x80x94C(xe2x95x90O)Oxe2x80x94, xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94, or xe2x80x94Si(CH3)2xe2x80x94, and/or
b) one CH2 group which is not adjacent to the ring system may be replaced by xe2x80x94Cxe2x89xa1Cxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94, and/or
c) one xe2x80x94CH2xe2x80x94 group may be replaced by cyclopropane-1,2-diyl, cyclobutane-1,3-diyl or cyclohexane-1,4-diyl or 1,3-dioxane-2,5-diyl, and/or
d) one or more H atoms may be replaced by F and/or Cl,
R7 is a straight-chain or branched alkyl radical having from 1 to 16 carbon atoms or a straight-chain or branched alkenyl radical having from 2 to 16 carbon atoms, in which one non-terminal CH2 group may be replaced by xe2x80x94Oxe2x80x94 and/or one CH2 group may be replaced by cyclohexane-1,4-diyl or phenylene-1,4-diyl,
R8 is hydrogen, a straight-chain or branched alkyl radical having from 1 to 12 carbon atoms or a straight-chain or branched alkenyl radical having from 2 to 12 carbon atoms, in which
a) one or more non-adjacent and non-terminal CH2 groups may be replaced by xe2x80x94Oxe2x80x94, xe2x80x94C(xe2x95x90O)Oxe2x80x94, xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94 or xe2x80x94Si(CH3)2xe2x80x94, and/or
b) one CH2 group may be replaced by xe2x80x94Cxe2x89xa1Cxe2x80x94, cyclopropane-1,2-diyl or cyclobutane-1,3-diyl, and/or
c) one or more H atoms may be replaced by F and/or Cl, or the -M3-A3-R10 group, in which
M3 is xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94CH2xe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94CH2xe2x80x94, xe2x80x94CF2xe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94CF2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94CF2xe2x80x94CF2xe2x80x94, xe2x80x94(CH2)4xe2x80x94, xe2x80x94OC(xe2x95x90O)CFxe2x95x90CFxe2x80x94 or a single bond,
A3 is 1,4-cyclohexylene, in which one or two H atoms may be replaced by CH3 and/or F, 1-cyclohexene-1,4-diyl, in which one H atom may be replaced by CH3 or F, or 1,3-dioxane-2,5-diyl,
R10 is hydrogen, a straight-chain or branched alkyl radical having from 1 to 12 carbon atoms or a straight-chain or branched alkenyl radical having from 2 to 12 carbon atoms, in which
a) one or more non-adjacent and non-terminal CH2 groups may be replaced by xe2x80x94Oxe2x80x94, xe2x80x94C(xe2x95x90O)Oxe2x80x94, xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94, or xe2x80x94Si(CH3)2xe2x80x94, and/or
b) one CH2 group may be replaced by xe2x80x94Cxe2x89xa1Cxe2x80x94, cyclopropane-1,2-diyl or cyclobutane-1,3-diyl, and/or
c) one or more H atoms may be replaced by F and/or Cl,
R9 is a straight-chain or branched alkyl radical having from 1 to 12 carbon atoms, in which
a) one or more non-adjacent and non-terminal CH2 groups may be replaced by xe2x80x94Oxe2x80x94, xe2x80x94C(xe2x95x90O)Oxe2x80x94, xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94, or xe2x80x94Si(CH3)2xe2x80x94, and/or
b) one CH2 group which is not adjacent to the ring system may be replaced by xe2x80x94Cxe2x89xa1Cxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94, and/or
c) one xe2x80x94CH2xe2x80x94 group may be replaced by cyclopropane-1,2-diyl, cyclobutane-1,3-diyl or cyclohexane-1,4-diyl or 1,3-dioxane-2,5-diyl, and/or
d) one or more H atoms may be replaced by F and/or Cl.
The provision of compounds of the formula (I) very generally considerably broadens the range of liquid-crystalline substances which are suitable from various applicational points of view for the preparation of liquid-crystalline mixtures.
In this connection, the compounds of the formula (I) have a broad range of applications. Depending on the choice of substituents, they can be added to other classes of compound in order, for example, to modify the dielectric and/or optical anisotropy of a dielectric of this type. They can also serve to optimize its threshold voltage and/or its viscosity.
Compounds of the formula (I) are particularly suitable for modifying the dielectric anisotropy (xcex94xcex5) even when added in small amounts.
The invention thus relates to compounds of the formula (I) and to the use of these compounds as components of liquid-crystalline mixtures and liquid-crystal mixtures comprising compounds of the formula (I).
The compounds of the formula (I) can be employed in various liquid-crystal mixtures. In the case of nematic mixtures, they are particularly suitable for xe2x80x9cactive matrix displaysxe2x80x9d (AM-LCDs) (see, for example, C. Prince, Seminar Lecture Notes, Volume I, p. M-3/3-M-22, SID International Symposium 1997, B. B: Bahadur, Liquid Crystal Applications and Uses, Vol. 1, p. 410, World Scientific Publishing, 1990, E. Lxc3xcder, Recent Progress of AMLCD""s, Proceedings of the 15th International Display Research Conference, 1995, p. 9-12) and xe2x80x9cin-plane switching displaysxe2x80x9d (IPS-LCDs), and in the case of smectic liquid-crystal mixtures, they are suitable for smectic (ferroelectric or antiferroelectric) displays.
Further components of liquid-crystal mixtures which comprise compounds of the formula (I) according to the invention are preferably selected from the known compounds having smectic and/or nematic and/or cholesteric phases. Mixture components which are suitable in this sense are mentioned, in particular, in WO 00/36054, DE-A-19 531 165 and EP-A-0 893 424, which are expressly incorporated herein by way of reference.
The invention therefore also relates to liquid-crystal mixtures which comprise at least one compound of the formula (I), preferably in an amount of from 1 to 40% by weight, based on the liquid-crystal mixture. The mixtures preferably comprise at least 3 further components. In addition, the invention also relates to electrooptical display elements (displays) which contain the mixtures according to the invention.
Preference is given to displays which contain nematic or smectic (ferroelectric or antiferroelectric) mixtures according to the invention in combination with active matrix elements.
The display elements (displays) according to the invention are usually constructed in such a way that a liquid-crystal layer is enclosed on both sides by layers which are usually, in this sequence starting from the LC layer, at least one alignment layer, electrodes and a limiting sheet (for example made of glass). In addition, they may contain spacers, adhesive frames, polarizers and, for color displays, thin color-filter layers. Further possible components are antireflection, passivation, compensation and barrier layers as well as electrical non-linear elements, such as thin-film transistors (TFTs) and metal-insulator-metal (MIM) elements. The construction of liquid-crystal displays has already been described in detail in relevant monographs (see, for example, E. Kaneko, xe2x80x9cLiquid Crystal TV Displays: Principles and Applications of Liquid Crystal Displaysxe2x80x9d, KTK Scientific Publishers, 1987).
Possible synthesis routes to compounds of the formula (I) are indicated by way of example in the following schemes, with other processes also being feasible and possible.
The following abbreviations are used:
n-BuLi n-butyllithium
DCC dicyclohexylcarbodiimide
DDQ 2,3-dichloro-5,6-dicyano-p-benzoquinone
DEAD diethyl azodicarboxylate (azodicarboxylic acid diethyl ester)
Diglyme diethylene glycol dimethyl ether
DMAP 4-(dimethylamino)pyridine
DME dimethoxyethane
DMF N,N-dimethylformamide
KOtBu potassium tert-butoxide
LICOR organolithium compound+potassium tert-butoxide
LiTMP lithium 2,2,6,6-tetramethylpiperidide
MTBE tert-butyl methyl ether
NMP N-methylpyrrolidone
4-TsOH 4-toluenesulfonic acid 
The starting materials (E1) required for the synthesis of (I) are known from the literature and are for the most part commercially available:
4-methylcyclohexanone [589-92-4]; 4-ethylcyclohexanone [5441-51-0];
4-propylcyclohexanone [40649-36-3];4-butylcyclohexanone [61203-82-5];
4-pentylcyclohexanone [61203-83-6];4-hexylcyclohexanone [5441-57-6];
4-heptylcyclohexanone [16618-75-0];4-(4-methylcyclohexyl)cyclohexanone [151772-66-6]; 4-(4-ethylcyclohexyl)cyclohexanone [150763-46-5];
4-(4-propylcyclohexyl)cyclohexanone [82832-73-3];
4-(4-butylcyclohexyl)cyclohexanone [92413-47-3];
4-(4-pentylcyclohexyl)cyclohexanone [87625-10-3];
4-(4-hydroxyphenyl)cyclohexanone [105640-07-1]
Cyclohexanes containing specific radicals, for example cyclopropyl or dimethylsilyl, can be obtained starting from cyclohexane-1,4-dione monoethylene ketal [4746-97-8].
The following starting materials (E2) are known from the literature and some are commercially available:
Xxe2x95x90H, L1=L3=F 3,5-difluorobenzaldehyde [32085-88-4]
Xxe2x95x90F, L1=L3=F 3,4,5-trifluorobenzaldehyde [132123-54-7]
Xxe2x95x90OCH3, L1=L3=F 3,5-difluoro-4-methoxybenzaldehyde [654-11-5]
Xxe2x95x90NCS, L1=L3=F 3,5-difluoro-4-isothiocyanatobenzaldehyde [243651-31-2]
Xxe2x95x90Br, L1=L3=F 4-bromo-3,5-difluorobenzaldehyde [135564-22-6]
Xxe2x95x90Cl, L1=L3=F 4-chloro-3,5-difluorobenzaldehyde can be prepared analogously to the bromine compound in accordance with Liq. Crystals 23, 575(1997).
Xxe2x95x90CF3, L1=L3=F can be prepared from 3,5-difluoro-4-trifluoromethylbenzoic acid [261945-09-9], which is known from the literature, by a Rosenmund reduction analogously to J. Am. Chem. Soc. 108, 2608(1986).
Xxe2x95x90OCHF2, L1=L3=F can be prepared from 3,5-difluoro-4-difluoromethoxybenzoic acid [176508-82-0], which is known from the literature, by a Rosenmund reduction analogously to J. Am. Chem. Soc. 108, 2608(1986).
Xxe2x95x90H, L3=L4=F 2,3-difluorobenzaldehyde [2646-91-5]
Xxe2x95x90F, L3=L4=F 2,3,4-trifluorobenzaldehyde [161793-17-5]
After steps a), b) and c) have been carried out, the compounds (Ia) corresponding to (Z3) with Xxe2x95x90F, (Ib) corresponding to (Z3) with Xxe2x95x90OCF3 and (Id) corresponding to (Z3) with Xxe2x95x90Cl are obtained directly. 
(Ic) corresponding to (Z4) with Xxe2x95x90CN can be obtained analogously from (Z3) with Xxe2x95x90Br by reaction with CuCN/NMP analogously to WO 91/05780.
(Ie) and (If) can be obtained from (Z3) with Xxe2x95x90Br (or Xxe2x95x90OCH3, as shown in Scheme 2) by Suzuki reaction with 3,4,5-trifluorophenylboronic acid {in the case of (Ie)} or 3,5-difluoro-4-trifluoromethoxyphenylboronic acid {in the case of (If)}.
(Ig) can be obtained in accordance with Scheme 1 by reacting a compound (Z3) with L2=F and L3=H with an electrophilic fluorinating agent, for example F-TEDA-BF4 analogously to Taylor et al., Tetrahedron 55, 12431 (1999).
Further compounds of the formula (I) can be prepared by functionalization of corresponding intermediates (Z3); shown by way of example in Scheme 2 for the case (Z3)=OCH3, shown by way of example in Scheme 3 for the case (Z3)=H or (Z3)=Br, if (E2) is 4-bromo-3,5-difluorobenzaldehyde. 
The further conversion of the functional derivatives of the compounds (I) in the respective schemes into the end compoundsxe2x80x94for example by esterification of the carboxylic acid, by Suzuki reaction of the boronic acid, by Wittig reactions of the aldehyde or acetalization (to dioxanes) of the aldehydexe2x80x94are carried out by methods which are familiar to the person skilled in the art.
The preparation of the benzoic acids, cyclohexanecarboxylic acids and phenylacetylenes, phenylboronic acids, bromobenzenes and 2-alkylpropane-1,3-diols which are needed for the syntheses in accordance with Schemes 1, 2 and 3 and are, for example, alkyl-, alkenyl- or alkoxy-substituted and are optionally additionally fluorinated, and their reactions are known to the person skilled in the art and are described, for example, in WO 96/00710, WO 96/30344, Liq. Cryst. 1995, 18, 1, Mol. Cryst. Liq. Cryst. 1991, 204, 43, Liq. Cryst. 1997, 23, 389, Synthesis 1996, 589, WO 92/11241, EP-A 0665825 and J. Mater. Chem. 1999, 9, 1669. Correspondingly substituted benzyl alcohols and (hydroxymethyl)cyclohexanes R3-A2-CH2OH can be obtained, for example, from the corresponding benzoic acids or cyclohexane-carboxylic acids R3-A2-COOH by reduction with lithium aluminum hydride (general working procedure: Organikum [Synthetic Organic Chemistry], VEB Deutscher Verlag der Wissenschaften, 15th Edn., Berlin, 1984, Chapter 7.3.4). Bromination thereof using phosphorus tribromide (analogously to J. Org. Chem. 1984, 49, 2534-2540) gives the benzyl bromides or (bromomethyl)cyclohexanes R3-A2-CH2Br. Subsequent reaction with triphenylphosphine in xylene gives the triphenylphosphonium bromides R3-A2-CH2xe2x80x94P(Ph3)Br. Correspondingly substituted benzaldehydes and cyclohexanecarboxaldehydes R3-A2-CHO are obtainable, for example, by reduction of the respective carboxylic acid ester R3-A2-COOR (Bull. Korean. Chem. Soc. 1999, 20, 1373) or oxidation of the abovementioned benzyl alcohols and (hydroxymethyl)cyclohexanes R3-A2-CH2OH (Tetrahedron Lett. 1968, 30, 3363).
The synthesis of specific radicals is carried out, for example, in accordance with DE-A-19 528 085, DE-A-19 532 292 and DE-A-19 654 487. Compounds of the formula (I) according to the invention with a 1-cyclohexene-1,4-diyl or 2-fluoro-1-cyclohexene-1,4-diyl or 4-fluoro-3-cyclohexen-1-yl unit are prepared as described in Liq. Cryst. 1997, 23, 69, DE-A-4 427 266, DE-A-19 607 996, DE-A-19 528 665 and EP-A-0 736 513. Regarding the synthesis of further specific radicals R1 and R2, reference is additionally made, for example, to EP-A-0 355 008 (for compounds with silicon-containing side chains), U.S. Pat. No. 4,798,680 (for optically active compounds with a 2-fluoroalkoxy unit), EP-A-0 552 658 (for compounds with cyclohexylpropionic acid esters) and EP-A-0 318 423 (for compounds with cyclopropyl groups in the side chain).
The invention is explained in greater detail by the following examples.